Welding of at least one object to be welded (hereinafter objects of welding) such as thick steel plates requires a high efficiency of welding operations. The submerged arc-welding method is widely adopted because of its high welding efficiency as compared with those of the other welding methods, which is brought about by its large heat input to the objects of welding.
However, the submerged arc-welding method is problematic in that, because of its large heat input to the objects of welding, quality degradation, i.e., the decrease in toughness is caused in the welding heat affected zone around the weld zone. The decrease in toughness occurring in the welding heat affected zone is particularly serious in steel products in service at low temperatures such as in cold districts and LPG storage tanks. Therefore, the submerged arc-welding method is not desirable for welding of steel products used in particular at low temperatures, in spite of the advantage of allowing welding of the objects of welding at a high efficiency.
With this problem in view, a high-speed and large current gas-shielded arc-welding method of steel with the use of a mixed gas of inert gases and active gases as a shielding gas is disclosed in Japanese Patent Publication No. 9,571/78 dated Apr. 6, 1978 (Japanese Patent Application No. 135,559/74), which comprises the steps of: using at least one low-alloy steel solid wire consumable electrode having a diameter of substantially from 3.0 to 6.4 mm; feeding a welding current of from 600 to 1,500 amperes; supplying a shielding gas at a rate of from 50 to 200 l/minute per consumable electrode to shield the resulting arc; and, carrying out welding at a speed of from 300 to 1,500 mm/minute with an arc voltage of from 23 to 36 volts (hereinafter referred to as the "prior art 1").
According to the welding method of the prior art 1, it is possible to weld the objects of welding at a high speed with a large current, with a small heat input into the weld zone, and thus to weld even steel products to be used at low temperatures at a high efficiency without causing quality degradation at the welding heat affected zone.
However, the welding method of the above-mentioned prior art 1 is problematic in that penetration of a molten metal into the weld zone is partially very deep and a welding defect is caused at the portion of this deep penetration. FIG. 1 is a schematic sectional view illustrating a welding machine employed in the welding method of the above-mentioned prior part 1 and an example of weld zone. In FIG. 1, 1 is a nozzle. 2 is a consumable welding electrode fed through the nozzle 1 in the axial direction thereof and from the lowermost end of the nozzle 1 toward objects of welding 3. 4 is a feeding port of a shielding gas, which is provided at the upper portion of the nozzle 1. 5 is an ejecting port of shielding gas opening at the lowermost end of the nozzle 1. A welding power source (not shown) is connected between the objects of welding 3 and the consumable welding electrode 2, thus producing an arc 6 between the tip of the consumable welding electrode 2 and the objects of welding 3. Also in FIG. 1, 7 is a molten metal produced on the objects of welding 3 by the heat of the arc 6. In case of the welding method of the above-mentioned prior art 1, penetration of the molten metal 7 tends to be deep because of the high current density and the strong concentration of the arc. Such a deep penetration of the molten metal 7 causes occurrence of cracks in the objects of welding 3. These cracks tend to easily occur particularly when welding an I-shaped narrow groove having a narrow groove width, of which the groove faces face parallelly each other.
As a method for efficiently welding the objects of welding having the above I-shaped narrow groove without causing a defective fusion of corners, an arc-welding method of a narrow groove is disclosed in Japanese Patent Provisional Publication No. 133,871/80 dated Oct. 18, 1980 (Japanese Patent Application No. 42,014/79), which comprises: feeding a consumable welding electrode through a rotatable nozzle, which is provided so that the center axis of said nozzle is located at the center of an I-shaped narrow groove having groove faces formed in the objects of welding facing parallelly each other, eccentrically from the center axis of said nozzle toward the weld zone of the objects of welding; producing an arc between the tip of said consumable welding electrode and said weld zone to weld the weld zone by the heat of the arc; rotating said nozzle to cause a circular movement of said arc corresponding to the eccentricity of said arc from the tip of said consumable welding electrode; and, feeding a shielding gas toward said weld zone to shield said arc and said weld zone from the open air (hereinafter referred to as the "prior art 2").
However, the above-mentioned prior art 2 discloses only a method for stably welding a narrow groove by solving the insufficient penetration at the both corners of the narrow groove, the conditions of which include a welding current of 300 amperes and a number of rotation of the nozzle of about 1,200 r.p.m., and which does not satisfy the requirements of a high-efficiency gas-shielded arc-welding free of welding defects.